I have AMD HD5850 and it work in LInux with opensource drivers better than 280 GTX!
If you want nvidia and its proprietary driver and can't/don't want/don't care for opensource radeon driver, I suggest you pick anything nvidia 4xx+, with at least 192bit GDDR5 bus.
Pre 4xx hardware is actually very hot and I might say inefficient compared to fermi/kepler. Its worth the money to puchase fermi/kepler today and sell 2xx/G20Xx.
On question if they are same - it depends.
Nvidia/AMD provide chips and standard PCB layout. OEMs either purchase existing stanard PCB designs or make their own. Example: ASUS Mars/Matrix etc.
Then, they pick quality of components they use on PCB - cheaper one have tendency to fail prior to higher quality ones. So, they are TWINS, but of different QUALITY.
For this, you need to look for reviews and look on PCB of specific model itself.